That is not easy. It works till the first drama and then some angry person deletes it all, or changes and takes the keys with them. So you have to really take care who to trust.
"If the project leadership does not see a value in using open infrastructure"
And I do not think he ever said that. He just said it was easier this way for the moment. Not that discord is meant to be the solution till eternity.
Simply put, this approach is proven to work. We can always "butwhatif?" ourselves into a corner and get paralyzed because of that, but it's senseless.
So I totally agree, that there are tons of examples where it worked and in the long term I also think they should switch to something better, but for the moment I see why they don't, as it is working for them as it is.
As for random volunteers disappearing, the protection against that is easy - increase the bus factor, by multiple people having necessary access, and by keeping your infrastructure documented. This should be table stakes for any project that becomes even marginally popular.
Then, if a random volunteer disappears, things might slow down due to lack of manpower, sure, but not come to a screeching halt.
You can't increase bus factor with Discord, since it's out of your control.